Rechargeable batteries are provided with a variety of protective functions for preventing damage to the battery caused by abnormal use such as overcharging, over discharging, short-circuiting between the positive and negative electrodes, or short-circuiting of the battery peripheral circuitry of the device in which the battery is used. Examples of these functions include PTC (Positive Temperature Coefficient) elements that prevent excessive discharge current due to short-circuiting or the like, and safety vents that release internal pressure by cutting off the power circuit in the event of an abnormal rise in internal pressure caused by overcharging. These protective functions are known as functions incorporated into the rechargeable battery itself. A lithium ion rechargeable battery is normally provided with a battery protection circuit to prevent overcharging, over discharging, and so forth. Such battery protection circuit is generally unitized with the battery as part of battery pack configuration, in which a circuit board that makes up the battery protection circuit is integrally contained in the pack case along with the lithium ion rechargeable battery. It is common knowledge that battery packs such as this, in which the rechargeable battery and the protection circuit are integrally housed in a case, are often used for battery-powered devices such as cellular telephones and laptop computers.
In addition to preventing overcharging and over discharging as mentioned above, this battery protection circuit can also have such functions as cutting off excessive current or monitoring cell temperature, and if this circuit could be integrated with the rechargeable battery itself, it would be possible to create a rechargeable battery equipped with a battery protection circuit without constituting a battery pack, which would make the rechargeable battery more universally applicable.
When a battery protection circuit is integrated with a rechargeable battery, however, it is difficult to avoid making the external dimensions larger or losing the original configuration of the rechargeable battery. In particular, a flat-shaped rechargeable battery is made thin so that it can be used in small devices such as portable devices, and when a battery protection circuit is integrated with such a rechargeable battery it is difficult to find space for the circuit board that makes up the battery protection circuit, which means that it is difficult to integrate a battery protection circuit without increasing the external dimensions.
Also, with a small, flat-shaped rechargeable battery, there is little space in which to provide the above-mentioned PTC element or current cut-off vent, so the elements for electromotive force must be reduced in size in order to ensure this space, the problem with which is that the battery capacity is diminished. Therefore a PTC element must be provided as part of the battery protection circuit outside the rechargeable battery, and the battery protection circuit must take over the function of the current cut-off vent. However, if a PTC element is provided as part of the battery protection circuit, then an increase in volume is inevitable when the battery protection circuit is integrated with the rechargeable battery, which is contrary to the objective of minimizing the increase in external dimensions. Also, reliability suffers when an ordinary battery protection circuit is made to take on the function of a current cut-off vent, making it difficult to obtain a level of reliability at which the rechargeable battery will definitely be protected.
It is an object of the present invention to provide a battery equipped with a battery protection circuit, with which a compact construction that can be applied to a flat-shaped rechargeable battery minimizes the increase in the external dimensions of the rechargeable battery, and with which the battery can be protected by enhancing the function of the battery protection circuit and without providing a PTC element or a current cut-off vent, and which can be used by itself as a battery, without a battery pack configuration being required.